Conventional second-generation mobile communication systems such as PDC (Personal Digital Cellular) have adopted a handover scheme in which a mobile station, when moving between cells while communicating, establishes a communication channel with a handover target cell after releasing a communication channel with a handover source cell (hard handover scheme).
On the other hand, conventional CDMA mobile communication systems have adopted a handover scheme in which a mobile station, when moving between cells while communicating, adds a communication channel with a handover new cell before releasing a communication channel with a handover current cell (soft handover scheme).
In this specification, a base transceiver station may be configured to manage one cell or may be configured to manage a plurality of cells.
In the soft handover scheme, a mobile station is configured to simultaneously establish a plurality of communication channels with a plurality of cells for communication in a given area near the boundary between the cells.
FIGS. 1(a) to 1(d) show information transmitting methods in the soft handover scheme. As shown in FIGS. 1(a) to 1(d), multiple information transmitting methods have been known.
In an uplink information transmitting method in the soft handover scheme shown in FIG. 1(a), a base transceiver station BTS receives information from a mobile station MS via a plurality of cells, and performs soft combining on the received information for transmission to a radio network controller RNC.
In another uplink information transmitting method in the soft handover scheme shown in FIG. 1(b), a plurality of base transceiver stations BTS1 and BTS2 receive information from a mobile terminal MS in their managing cells, respectively, and transmit the received information to a radio network controller RNC together with reliability information corresponding to reception qualities (such as received power, received SIRs, received error rates, or CRC check results). The radio network controller RNC performs selective combining to select information of the highest reliability among the information from the base transceiver stations BTS1 and BTS2, based on the reliability information from the base transceiver stations BTS1 and BTS2, and transmits the information after selective combining to an upper node (not shown).
A downlink information transmitting method in the soft handover scheme shown in FIG. 1(c), a base transceiver station BTS distributes information from a radio network controller RNC and transmits the same information to a plurality of cells. A mobile station MS receives the same information via the plurality of cells and performs soft combining on the received same information.
In another downlink information transmitting method in the soft handover scheme shown in FIG. 1(d), a radio network controller RNC distributes information from an upper node and transmits the same information to a plurality of base transceiver stations BTS1 and BTS2. A mobile station MS receives the same information from a plurality of cells (base transceiver stations BTS1 and BTS2) and performs soft combining on the received same information.
In the conventional mobile communication systems, broadcast communication has been known, in which, as shown in FIG. 2, one or more base transceiver stations BTS simultaneously transmit the same information to an unspecified large number of mobile stations MS in a given cell(s) (e.g., see non-patent literature 1).
Also, in the conventional mobile communication systems, multicast communication has been known, in which, as shown in FIG. 3, the same information is transmitted to a plurality of mobile stations MS belonging to a particular group (e.g., see non-patent literature 2).
In the broadcast communication and multicast communication, in order to reduce redundancy and loss of received information at a mobile station MS due to movement between cells, a method of transmitting the same information by performing timing synchronization with a plurality of cells has been conceived.
(Non-Patent Literature 1)
3rd Generation Partnership Project Technical Specification Group Terminals, 23.041 Technical realization of Cell Broadcast Service (CBS), October, 2000
(Non-Patent Literature 2)
3rd Generation Partnership Project Technical Specification Group Radio Access Network, 25.324 Broadcast/Multicast Control BMC, December, 2000
In the conventional mobile communication systems as described above, however, a mobile station MS cannot check the accuracy of timing synchronization or an information receiving method (e.g., soft combining, selective combining, or no combining processing) for transmission of the same information from a plurality of cells, and cannot select an optimum information receiving method. This causes the problem of an increase in the transmission power of a base transceiver station BTS due to deterioration of reception quality at the mobile station MS, or redundancy or loss of received information at the mobile station MS.
The present invention has been made in view of the above problem, and has an object of providing a mobile communication system, a mobile station and a radio network controller which enable selection of an information receiving method at a mobile station MS so as to avoid an increase in the transmission power of a base transceiver station BTS or redundancy or loss of received information at the mobile station MS in broadcast communication or multicast communication.